This is a proposal to study the structure, connections, and neurochemical organization of the primary auditory cortex (AI), medial geniculate body, and inferior colliculus in several species. This work aims at defining the principal types of neurons participating in corticifugal and thalamocortical relations, and the cells projecting between different cortical areas or within single layers in AI. These pathways play a critical role in the normal flow of auditory information related to sound localization or the analysis of speech, and damage to them produces a spectrum of neurobehavioral dysfunctions not limited to purely auditory deficits. In a series of experiments in rats, cats, bats, and monkeys several questions are considered using axoplasmically transported tracers or immunocytochemical techniques; each inquiry is directed primarily to one species, and may include others for comparative purposes since previous work finds parallels in auditory organization as well as species-specific differences with important functional implications. The experiments include studies of (i) thalamocortical terminations in AI, including their laminar targets, form, and transmitter-specific labeling of geniculorecipient neurons; (ii) the types of commissural neurons participating in physiologically-defined binaural cortical subregions; (iii) patterns of ipsilateral corticocortical projections between AI and the non-primary fields; (iv) the local connections within single AI laminae, and between subdivisions of AI; (v) the targets and structure of AI axons projecting to the inferior colliculus; (vi) the populations of cortical neurons immunoreactive for putative transmitters, such as gamma- aminobutyric acid or glutamic acid decarboxylase, and glutamate; (vii) the identification of the transmitter(s) of thalamocortical projection and intrinsic neurons; (viii) the comparative analysis of GABAergic inferior colliculus neurons and axon terminals; and (ix) the form of auditory dendrites in several species at different levels of the auditory pathway. The goal of this work is a clearer picture of the main neural circuits implicated in binaural pathways or frequency analysis of sound, and the nature of species differences and homologous neurons. Such patterns embody the larger questions of parallel and hierarchical processing in AI and in the primary visual and somatic sensory cortices, and are critical to a more developed picture of cortical function.